I usually just go "yeah", "busy", or "occupied". Worst ones are at some of the public restrooms on my island, for somereason people have this compulsion to rip the doors or locks of the stalls. I've more than a few time had to use my knee to bar the door so that when someone checks they know it's taken, I've also found just whistling seems to give a clue as well.

Don't you hate it when there's a whole row of unoccupied toilets or worse urinals and someone takes the one next to you and trys to start a conversation? Freaking wierd man!